The topic of the present study is the establishment of a highly sensitive method to detect the structural modification of DNA caused by the bindings of carcinogens. Electron microscopy, gel electrophoresis and stepwise digestion with specific nucleases of double stranded circular DNA provided evidence that the binding of carcinogens to different DNA bases and phosphate groups resulted in different types of modifications. (2) The effect of the binding and modification of DNA on its replication could be studied with already established in vitro assay systems using plasmid Col E1 DNA and E. coli cell extract. Factors involved in the replication of normal and carcinogen damaged DNA and its repair are under investigation.